Josh, don't overlook what LinkedIn has done with its professional network. They have done a nice job of monetizing the process for making connections within the network (services for job seekers and hiring managers) and for observing the network from the outside (market research).
This approach is pretty specific to LinkedIn at the moment but I suspect it is applicable to any narrowly defined social network where "narrow" refers to either the members or the focus. I could see similar monetization mechanics in place for a physician network, to name the first that comes to mind.

by Josh Bernoff We're arguably 10 years into the social network phenomenon (Friendster was founded in 2002). By now we should know the main business models. But popular sites like Tumblr (no visible means of support) raise the question, are there really any other good ways to make money other th...